Simulating the effects earthquakes had on the Canterbury Television building would have cost "several million", an inquiry has heard.

Dr Alan Reay, the engineer whose company designed the building, last week told a royal commission hearing he was "dissatisfied" with a Department of Building and Housing report on why the building collapsed in the February 22, 2011, quake.

More experiments, such as a reduced-scale model of the CTV building being tested on a shake table, should have been done, he said.

Today, under questioning from commissioner Richard Fenwick, Reay said the cost of that testing would have been "several million" dollars.

Fenwick suggested modelling the shaking would not be a "simple matter".

"You've this building, half-scale, weighing hundreds of tonnes and put on a shake table. These shake tables are enormously expensive to run," he said.

Last week, Reay presented to the commission five collapse scenarios in response to the DBH findings.

Commission chairman Justice Mark Cooper questioned why Reay had not provided calculations in his report or studied the building plans.

"I was putting forward scenarios I considered hadn't been considered. I wasn't analysing those and coming up with a definitive answer," Reay said.

"It could be that some of them aren't significant in relation to the collapse of the building. In preparing a report such as the DBH prepared, I would have expected some investigation and analysis of those [alternative scenarios]."

Reay earlier expressed concerns about the inadequate time given to respond to the report.

Justice Cooper said Reay had six months between the report and the hearing to consider the other possible explanations for the collapse.

Missing documents unlikely to be recovered

The commission lawyer for victims' families, Marcus Elliot, earlier asked Reay why key documents requested by the commission had not been provided.

Reay said not all files relating to the CTV building were retained.

"We had no legal obligation to retain job files ... It was just us retaining what was appropriate," he said.

Some company files held in an off-site storage facility had been ruined by a leaking roof, he said.

Documents later supplied to the commission were found in a storage box marked "miscellaneous".